Crude oil or natural gas produced in recent years contains wet hydrogen sulfide (H2S). Therefore, hydrogen embrittlement derived from the hydrogen sulfide is a problem in oil country tubular goods for use in drilling an oil or natural gas well or line pipes for transporting produced crude oil or natural gas. The hydrogen embrittlement includes sulfide stress cracking (hereinafter simply as “SSC”) caused in a steel product under static external stress and hydrogen induced cracking (hereinafter simply as “HIC”) caused in a steel product with no external stress thereupon.
The oil country tubular goods have ends in a screw joint form. A plurality of oil country tubular goods are coupled with each other by their screw joints and assembled in the vertical direction of an oil or gas well. At the time, the oil country tubular goods are subjected to tensile stress by their own weight. Therefore, it is particularly required that the oil country tubular goods have SSC resistance. As oil wells have come to be more deeply drilled in recent years, the oil pipes must have even higher SSC resistance. In order to improve the SSC resistance, steel may be cleaned, the martensite ratio in the steel product may be increased, or the microstructure of the steel product may be refined.
Meanwhile, a plurality of line pipes are coupled with each other by welding and assembled basically in the horizontal direction, and therefore no such static stress as the case of the oil country tubular goods is imposed on the line pipes. Therefore, it is required that the line pipes have HIC resistance.
It is believed that HIC is caused by gas pressure generated when penetrating hydrogen accumulated at the interface between MnS elongated by rolling and a base material turns into molecular hydrogen. Therefore, in order to improve the HIC resistance of a line pipe, the following two conventional anti-HIC measures (first and second anti-HIC measures) have been taken. Many such anti-HIC measures have been reported for example as those in Japanese Patent Laid-Open Nos. 6-271974, 6-220577, 6-271976, and 9-324216.
In the first anti-HIC measures, the resistance of steel against hydrogen embrittlement is increased, details of which are as follows.
(1) To highly purify and clean the steel. More specifically, S is reduced as much as possible in during making the steel, so that the amount of MnS in the steel is reduced.
(2) To reduce macro center segregation.
(3) To control the form of sulfide inclusions (type A inclusions) by adding Ca. More specifically, the form of the sulfide-based inclusions is changed by Ca treatment from the form of MnS to the form of CaS that is harder to be elongated during hot-rolling.
(4) To control the microstructure by controlled rolling followed by accelerated cooling. More specifically, an original plate for steel pipe is subjected to controlled rolling and accelerated cooling. In this way, the microstructure of the original plate can be homogeneous and the hydrogen embrittlement resistance can be improved.
(5) To reduce Mn segregation and P segregation in the steel.
(6) To reduce type B inclusions such as alumina in the steel.
A number of specific methods of producing a steel product for use as a line pipe provided with these first anti-HIC measures have been reported for example as those in Japanese Patent Laid-Open Nos. 2003-13175 and 2000-160245.
In the second anti-HIC measures, hydrogen is prevented from penetrating the steel, details of which are as follows.
(7) To prevent hydrogen from penetrating the steel in a wet hydrogen sulfide environment by adding Cu.
(8) To prevent hydrogen from penetrating the steel by adding an inhibitor (corrosion inhibitor) or coating the surface.
However, the line pipes provided with the above-described, well known anti-HIC measures still suffer from HIC. Therefore, there have been further attempts to improve the HIC resistance.